In recent years, the importance of pipelines as a method for long distance transport of crude oil, natural gas, etc. has been increasingly rising. Further, 1) to improve the transport efficiency by raising the pressure and (2) to improve the field installation ability by reducing the outside diameter and weight of line pipe, line pipe which has higher strength is being used in increasing instances. At the present, high strength line pipes of up to the American Petroleum Institute (API) standard X120 (tensile strength 915 MPa or more) have been put into practice. These high strength line pipes are generally produced by the UOE method, bending roll method, JCOE method, etc.
However, for trunk line pipe for long distance transport use, line pipe corresponding to the API standard X60 to X70 continues to be used in large numbers. As line pipe corresponding to the X60 to X70, much spiral steel pipe and electric resistance welded steel pipe with their high field installabilities are being used.
As the material which is used for the production of line pipe, when using the UOE method, bending roll method, or JCOE method to produce the line pipe, hot rolled steel plate which is not wound in a coil shape is used. On the other hand, when producing spiral steel pipe or electric resistance welded steel pipe, hot rolled steel plate which has been wound in a coil shape is used. Here, hot rolled steel plate which is not wound in a coil shape will be referred to as “plate” while hot rolled steel plate which is wound in a coil shape will be referred to as a “hot coil”.
PLT's 1 to 10 describe hot coils which are used for the production of spiral steel pipe or electric resistance welded steel pipe. Further, PLT's 11 to 14 describe plates which are used when using the UOE method, bending roll method, or JCOE method to produce line pipe.
Line pipe which transports crude oil, natural gas, or other flammable material require reliability at ordinary temperature of course and also reliability at low temperatures since it is used even in arctic regions. Therefore, the plate and hot coil which serve as materials for thick line pipe are required to be reduced in variation of ordinary temperature strength and to be improved in low temperature toughness.
The plates which are described in PLT's 11 to 14, since there is no coiling step, are large in freedom of conditions for cooling the steel plate after hot rolling and can give stable, uniform steel structures. Further, since there is no coiling step, sufficient time can be taken for holding the steel plates at the recrystallization temperature range between the rough rolling and finish rolling, so from this as well, the desired steel structure can be stably obtained. As a result, the plates which are described in PLT's 11 to 14 are small in deviation in ordinary temperature strength and excellent in low temperature toughness as well.
On the other hand, the hot coils which are described in PLT's 1 to 10 are not sufficiently reduced in deviation in ordinary temperature strength and are not sufficiently improved in low temperature toughness either. PLT's 1 to 10 describe cooling methods for steel plate after hot rolling so as to reduce the deviation in strength of the hot coils and improve the low temperature toughness. In particular, PLT's 1 to 2 and 6 to 9 describe cooling steel plate after hot rolling in multiple stages. However, in the production of a hot coil, there is a coiling step and the rough rolling and finish rolling are performed consecutively, so the restrictions on the production conditions become greater. Therefore, with just the improvements of the cooling method which are described in PLT's 1 to 10, the desired steel structure was not obtained and it was difficult to obtain hot coil with little deviation in ordinary temperature strength and excellent in low temperature toughness.